Many kinds of electrolytic solutions containing an electrolyte such as KOH, LiBF.sub.4 or LiClO.sub.4 dissolved in water or an organic solvent have hitherto been practically used in electrochemical devices such as electric batteries, display elements and sensors. These electrolytic solutions have high ion conductivity and come into good contact with electrodes.
There is, however, always a possibility that electrochemical devices produced with an electrolytic solution may cause liquid leaking because the electrolytic solution is in a liquid state; they are, therefore, required to have a structure for sealing the electrolytic solution therein. Further problems have been found in these electrochemical devices, such as difficulties in the production of sheet-shaped lightweight devices.
In recent years, various inorganic solid electrolytes have been known, such as RbAg.sub.4 I.sub.5, Na.sub.2 O.MgO.5Al.sub.2 O.sub.3, Na.sub.2 O.5Ga.sub.2 O.sub.3, Na.sub.2 O.11Al.sub.2 O.sub.3 and Li.sub.3 N. These inorganic solid electrolytes can solve the above problems of electrochemical devices, such as liquid leaking. They, however, have different problems such as weak adhesion to electrodes and difficulties in the formative working.
Much attention has been paid to polymer solid electrolytes as the material for electrochemical devices, which may solve all these problems. The use of a polymer solid electrolyte changes the above electrochemical devices into all solid-state devices having no fluidity as derived from electrolytic solutions. In addition, the viscoelasticity of a polymer in the polymer solid electrolyte can solve the above problems on the adhesion to electrodes and the workability. The polymer solid electrolyte also serves, when disposed between electrodes, as a separating film for these electrodes, and can be formed to have a thickness of 1 mm or less. For these reasons, there has been a great demand for polymer solid electrolytes with higher efficiency for electrochemical devices, and many studies for their development have been extensively made in the art.
For example, in the production of a secondary battery, the use of an inorganic solid electrolyte may cause a change in the shape of a positive electrode active material of the battery with the progress of an electric charge or discharge reaction, which gives a strain at the electrode-and-electrolyte interface and causes a lowering in the performance of the secondary battery. In contrast, polymer solid electrolytes themselves have flexibility, so that they can follow any change in the shape of a positive electrode active material and can have satisfactory electric charge and discharge characteristics. The polymer solid electrolytes can also be integrated to a high degree by making them thinner and they have, therefore, been expected to find applications as the material to produce power supplies for automobiles or for domestic use.
Furthermore, when a conventional electrolytic solution is used in a secondary battery using metal lithium, repeated cycles of electric charge and discharge make lithium dendrites formed in the electrolytic solution. The formation of dendrites causes serious problems such as short circuit and rupture of the secondary battery; therefore, no metal lithium secondary batteries have been put to practical use. In contrast, when a polymer solid electrolyte is used in a secondary battery using metal lithium, the polymer solid electrolyte prevents the formation of lithium dendrites or gives no such formation at all; therefore, polymer solid electrolytes have been greatly expected as the material for metal lithium secondary batteries.
Electrochromic displays are known as another example of the electrochemical device using an electrolytic solution, an inorganic solid electrolyte or a polymer solid electrolyte. The electrochromic displays exhibit the behavior of reversible color development and extinction by the electrochemical oxidation-reduction reaction. The electrochromic displays have excellent advantages such as no influence of display angles, good memory properties, simple cell structure and various color tones; they have, therefore, been expected as display elements that can be substituted for liquid crystal display elements.
In the electrochromic display, for example, a polymer solid electrolyte having high ion conductivity in the shape of a film can be used in combination with a layer made of a material exhibiting the behavior of reversible color development and extinction (e.g., tungsten oxide, Prussian blue, phthalocyanine complex with cobalt). The electrochromic display produced with such a combination is expected to make a good response of color development and extinction.
The polymer solid electrolyte also makes possible the design of all solid-state electrochromic displays in which the adhesion between electrodes and films can always be kept at a high level. The electrochromic displays using polymer solid electrolytes may, therefore, be used under various environments (e.g., those giving continuous vibration, those giving large strain deformation) as compared with the conventional ones.
Electric double layer condensers are known as still another example of the electrochemical device using an electrolytic solution, an inorganic solid electrolyte or a polymer solid electrolyte. The electric double layer condensers have the advantage of having an electrostatic capacity equal to those of the ordinary secondary batteries, in addition to the original advantages of condensers, i.e., long life characteristics, and rapid electric charge and discharge characteristics. In particular, for the electric double layer condensers, much interest has been increasingly given to the production of all solid-state electric double layer condensers from the viewpoints of making them thinner, improving the productivity and preventing the leaking of any liquid component such as an electrolytic solution. In the production of such all solid-state electric double layer condensers, the production of a polymer solid electrolyte having an ion conductivity equal to that of an electrolytic solution has been desired to improve the adhesion to electrodes and to keep the rapid electric charge and discharge characteristics.
Many studies have been made of polymer solid electrolytes. For example, a polymer solid electrolyte with a polyether such as polyethylene oxide is described in J. Amer. Chem. Soc., 21, 648(1988). This is a solid electrolyte of the type (polyether type) in which ions entrapped in the polymer chains migrate according to the thermal motion (segment motion) of the polymer chains.
In the polymer solid electrolytes of the polyether type, however, ion conductivity at around room temperature, which is generally most required, has been difficult to become higher than 10.sup.-4 S/cm.
Therefore, an improvement of the ion conductivity at around room temperature has required the use of a lower molecular weight polyether or the softening of a polyether itself. Such a lowering in the molecular weight or softening has deteriorated the mechanical strength of a polymer solid electrolyte and made it difficult to withstand actual use.
Some polymer solid electrolytes made of a polar polymer such as polyacrylonitrile and an electrolytic solution containing an electrolyte dissolved in an organic solvent (i.e., so-called polymer electrolyte gels) are described in J. Polym. Sci., 27, 4191(1982), J. Polym. Sci. Polym. Phys. Ed., 21, 939(1983), J. Electrochem. Soc., 137, 1657 (1990), JP-A 4-306560, JP-A 7-45271 and JP-A 7-82450. A polymer electrolyte gel made of a polar polymer such as poly(methyl methacrylate) and an electrolytic solution containing an electrolyte dissolved in an organic solvent is disclosed in JP-B 58-56467. Furthermore, a polymer electrolyte gel made of an acrylonitrile copolymer, a polyalkylene oxide and an electrolytic solution containing an electrolyte dissolved in an organic solvent is disclosed in JP-A 7-37419.
These polymer electrolyte gels have relatively higher ion conductivity than that of the above polymer solid electrolytes with polyethers; they, however, contain a polar polymer in a concentration of about 12 wt % or higher, so that the amount of an electrolyte that can dissolve in the electrolytic solution is inevitably decreased and they have an ion conductivity only about one fifth that obtained by the single use of an electrolytic solution.
The ion conductivity of such a polymer electrolyte gel may be improved by decreasing the concentration of a polar polymer contained therein and thereby increasing the concentration of an electrolyte. However, if the concentration of a polar polymer is about 10 wt % or lower, the polymer electrolyte gel cannot retain the shape in the range of temperature enough to withstand practical use (e.g., -20.degree. C. to 80.degree. C.; this is the temperature range found in automobiles). At a temperature ranging from 50.degree. C. to 80.degree. C., for example, some problems arise, such as liquefaction of the polymer electrolyte gel and separation between the polar polymer and the electrolytic solution because the polar polymer in the polymer electrolyte gel cannot carry the electrolytic solution. Thus, the advantages of the polymer electrolyte gel are lost at high temperatures, so that the polymer electrolyte gel exposed to high temperatures becomes unstable for long-term use and cannot be provided with satisfactory reliability.
The polymer electrolyte gel with a polar polymer is produced, for example, by casting. The casting is achieved by a process (of volatilization and concentration) in which a polar polymer is dissolved in an electrolytic solution containing a volatile organic solvent such as acetonitrile (or a polar polymer is dissolved in an ordinary electrolytic solution, which is then diluted with a volatile organic solvent), and the resulting solution is then spread over the surface of a flat plate, and the volatile organic solvent is evaporated under reduced pressure with heating or under atmospheric pressure to give a concentrated polymer-containing electrolytic solution, thereby causing gelation. This process, however, has serious problems of safety, such as outbreak of fires and induction of toxic symptoms in handling persons, because the step of evaporating a volatile organic solvent is involved as an essential step, and this process further has its own problem that polymer electrolyte gels cannot be produced in the shape of a thin film having a large surface area; therefore, this process cannot be practically applied to industrial use.
The polymer solid electrolyte containing a polymer electrolyte gel, when used in portable instruments such as thin-shaped batteries, is generally required to have a thickness of 500 .mu.m or less, preferably 100 .mu.m or less. In addition, the polymer solid electrolyte, when used in electrochemical devices such as sensors, is not particularly limited to the shape of a film, but is required to have workability into various shapes.